Internal combustion engines combust an air/fuel (A/F) mixture within cylinders to drive pistons and to provide drive torque. Air is delivered to the cylinders and an intake manifold via a throttle. A fuel injection system supplies fuel from a fuel tank to provide fuel from a desired A/F mixture to the cylinders. To prevent release of fuel vapor, vehicles also typically include an evaporative emissions system, which includes a canister that absorbs fuel vapor from a fuel tank, a canister vent valve and a purge valve. The canister vent valve allows air to flow into the canister. The purge valve supplies a combination of air and vaporized fuel from the canister to the intake system.
Closed-loop control systems adjust inputs of a system based on feedback from outputs of the system. By monitoring the amount of oxygen in the exhaust, closed-loop fuel control systems manage fuel delivery to an engine. Based on the output of oxygen sensors, the engine control module adjusts the fuel delivery to match the ideal A/F ratio (14.7 to 1). By monitoring the engine speed variation at idle, closed-loop speed control systems manage engine intake airflows and spark advance.
Under some circumstances, liquid fuel rather than fuel vapor can be present in the canister. Controlling the fuel system when liquid fuel is present in the canister can be a difficult task. Liquid fuel in the canister can produce high engine emissions, undesirable idle surge, steady throttle surge, or engine stall. If this problem occurs, a vehicle may also fail evaporative emissions requirements.